Ground-based Observational Support of the Fermi Gamma-ray Space Telescope at the University of Arizona

Overview of the Optical Observational Program

The launch of the Fermi Gamma-ray Space Telescope on June 11, 2008 began a new era in the study of high-energy emission from active galactic nuclei (AGNs). For the first time, a sizable number of AGNs can be monitored with unprecedented time resolution over a tremendous range in gamma-ray energies. The Large Area Telescope (LAT) aboard Fermi essentially views the entire sky every 3 hours in the energy range of 30 MeV -- 300 GeV. This allows for uniform and long-term monitoring of blazars, which are AGNs that were found to dominate the gamma-ray sky by the Compton Gamma Ray Observatory, the forerunner to Fermi.

Since blazars are highly variable on very short time scales (days or shorter) at all accessible wavelengths, it is important that supporting observations be obtained for these AGNs across the electromagnetic spectrum throughout the Fermi mission. Such observations will place the observed gamma-ray behavior of these extreme objects in context with the variability of the continuum at lower energies. Only through this multi-wavelength approach can a comprehensive understanding of the source structure and nonthermal emission mechanisms in blazars emerge. To help realize this goal, Steward Observatory of the University of Arizona contributed optical data for the LAT-monitored blazars and Fermi Targets of Opportunity (ToOs) during the first decade of its mission as part of the Guest Investigator Program.

As it has since the launch of Fermi, the primary contribution of the Steward Observatory program was monitoring the optical linear polarization of the blazar sample. These types of observations are particularly important for these AGNs because polarization provides the only direct information about the magnetic field within the region producing the optical synchrotron emission. The observations yield measures of both the degree of ordering and the orientation (projected onto the sky) of the magnetic field at the highest frequencies currently possible since there are no space-based capabilities to measure polarization at gamma-ray, X-ray, or ultraviolet wavelengths. As a bonus, the spectropolarimetry of the objects also yield measurements of the brightness and spectral index of the optical synchrotron light. An attempt was made to observe as many accessible gamma-ray-bright blazars at least once per night during the time allocated for this project on Steward Observatory telescopes. In addition, suitable ToOs that were brought to our attention by the Fermi project and others were also be observed. The program provides data to study possible connections between the gamma-ray flux and variability and the optical behavior observed. Correlations detected between these two spectral regions provide important information and constraints on models for the high-energy gamma-ray emission from the relativistic jets of these AGNs.

This project was funded through Fermi Guest Investigator Program grants NNX08AW56G, NNX09AU10G, NNX12AO93G, and NNX15AU81G. Please see here for properly crediting this program if measurements in the public Fermi/Steward Observatory blazar data archive are used for your research.

Personnel and Contact Information

Telescopes and Instrumentation

Observations were made using these Steward Observatory facilities:

All observations were obtained using the SPOL CCD Imaging/Spectropolarimeter.

Optical Observing Campaigns

Weather Information: See if there is any chance of a successful observation of your favorite blazar/ToO on the night that you are interested in.

Object Lists


Statement on data use and acknowledgment.

Fully reduced optical polarimetry, photometry, and spectra are available. Data are sorted by both date and object. They include:

There are explanatory README files for polarization, photometric, and spectroscopic data (also see arXiv:0912.3621).

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Please contact Paul Smith ( if you have questions concerning these web pages.

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